I'm curious- when the circuit is finished, will it be something that could be made so that different transistor types could be switched between, as found in VFE pedals for example?
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GK rails in a pedal?
- Thread starter sprag
- Start date
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
IMO, any lower hfe BJT should sound pretty much the same in this circuit so I wouldn't really bother with that.
Hey John, the V2 circuit has been updated and I changed the foldback circuit to work with a 1K ohm load. The conventional method works much better with small emitter resistors and 4-8 ohm speaker loads. I tried the original transistors for Q3 and Q4 in spice and didn't see much of a change at all... so you should be able to use any GP NPN's and PNP's that you have on hand that are at least 500mW.
Before finding that missing bias resistor (R21), changing the value of R11 made significant changes. With R11 at 10K, 4.7K and 22K there is very little change in operation, so that missing 1K bias resistor (R21) sets the Q3 and Q4 driver to the original GK specs. The foldback is set for 6mA peak load current for the 1K load of the R3 resistor or the 1K load of the primary of the tranny. You may want to install S2 to switch the foldback in and out of the circuit.
This circuit works really well in spice, so I think it's ready to go.
-Frank
Does anyone know if the HV power supply in these amplifiers sags under heavy load? I'm thinking that maybe part of this GK 'rails' sound might be the power supply for the power amplifier sagging when driven hard.
-Frank
-Frank
One more thought John... For the 100W version of this GK amplifier it takes about a 1Vpeak input signal to have the output drive to the 60V rails. Since this simulated amp has a much lower supply (+/-9V), you may want to adjust the AC gain so that the same input signal will drive this one to the rails. If you change R16 to 12K the AC gain will be a better match of the 100W GK amp hitting the rails.
-Frank
-Frank
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
thanks Frank. I think that it might be best to socket that resistor (R16) so I can mess around with its value and see what values tune it in better.
Last edited:
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
okay. I have a PCB drawn up for it. when I get back from running my errands, i'll etch it, build it up and test it later today.
John, hold off etching for a few more ideas and one small oversight. I'm leaving for a dinner party, but I will be back in a few hours.
-Frank
-Frank
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
Hey John, I am working on the correction to the schematics for the local positive feedback in the driver circuit that I had overlooked before.
But what I thought may be an issue earlier, is a problem for the output circuit.... the LF353 has phase reversal when it clips with a hot input. So the 353 is perfectly fine in the input position, but the original driver circuit does not have this issue. I didn't see this before because I wasn't using crazy hot signals... So I'm adding some extra circuitry to prevent the phase reversal issue in the output circuit. I will have the schematics done within the hour.
I have tested this every which way I can think of and think all of the bases are covered now.
-Frank
But what I thought may be an issue earlier, is a problem for the output circuit.... the LF353 has phase reversal when it clips with a hot input. So the 353 is perfectly fine in the input position, but the original driver circuit does not have this issue. I didn't see this before because I wasn't using crazy hot signals... So I'm adding some extra circuitry to prevent the phase reversal issue in the output circuit. I will have the schematics done within the hour.
I have tested this every which way I can think of and think all of the bases are covered now.
-Frank
You guys are mad scientists for sure!!!! Can't wait to hear how it turns out
It is incredibly awesome what you guys are doing. I can't wait to see what comes out of this.
Unqualified opinion but I believe it may- I used to own a Tech21 Boost/Distortion, a pedal with a sag control, and while it was unsuitable for bass due to extensive low end loss, when I tried it out I was quite struck by GK-like the reactiveness of it was, unless the sag was turned off.
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
cool Frank. let me know when you have everything updated and i'll make the corrections to the PCB layout.
I am going to delete the V1 and V2 schematics since John K is kind enough to build this circuit and give us feedback for any tweaks that may be needed. We will only work on this V3 and everything discussed about V1 should just be ignored. Some discussion before this post about the V2 will not make sense, because there have been a lot of changes to get to this point. Here is V3.
****THIS IMAGE HAS BEEN REMOVED. WE ARE WORKING ON A LATER VERSION NOW.****
Added C7 and R22 to match the positive feedback in the original 1982 GK power amp.
Removed the output protection diodes to the power rails, because they did absolutely nothing for this circuit. They are only necessary in the original power amp.
Changed the simulated foldback resistor values (R19 and R20) to clip the output signal at +/-3.5V.
Added D4 and D5 LED’s to reduce the supply voltage to the output transistors so that they clip before U1b. This keeps U1b from interfering with the clipping characteristics that we are interested in from Q1 and Q2. NOTE: U1 is powered differently from the previous schematics.
R23 and D8 keep U1b from phase inversion when driven hard.
Changed VR2 to 1K so that the level control should have a more useable range.
All diodes are 1N4002 rectifiers… just because the original GK amp used them.
I said I was going to post images of simulated waveforms, but because I had to break this circuit down into parts for simulation they wouldn’t make sense to most people. To avoid confusion, I will just make changes to the schematic per John K’s recommendations and observations.
Note: Some reference numbers for the parts have changed in this version.
-Frank
****THIS IMAGE HAS BEEN REMOVED. WE ARE WORKING ON A LATER VERSION NOW.****
Added C7 and R22 to match the positive feedback in the original 1982 GK power amp.
Removed the output protection diodes to the power rails, because they did absolutely nothing for this circuit. They are only necessary in the original power amp.
Changed the simulated foldback resistor values (R19 and R20) to clip the output signal at +/-3.5V.
Added D4 and D5 LED’s to reduce the supply voltage to the output transistors so that they clip before U1b. This keeps U1b from interfering with the clipping characteristics that we are interested in from Q1 and Q2. NOTE: U1 is powered differently from the previous schematics.
R23 and D8 keep U1b from phase inversion when driven hard.
Changed VR2 to 1K so that the level control should have a more useable range.
All diodes are 1N4002 rectifiers… just because the original GK amp used them.
I said I was going to post images of simulated waveforms, but because I had to break this circuit down into parts for simulation they wouldn’t make sense to most people. To avoid confusion, I will just make changes to the schematic per John K’s recommendations and observations.
Note: Some reference numbers for the parts have changed in this version.
-Frank
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If power supply sag is part of that GK 'rails' sound then there is much more work to be done.
-Frank
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
thanks for posting V3 Frank.
i'm gonna redraw my layout now. an i think that it's a good idea on the led's feeding the trannys.
I have a feeling that power supply sag possibly does have an effect on the 'rails' growl but maybe we should at least start with your V3 and then go from there.
i'm gonna redraw my layout now. an i think that it's a good idea on the led's feeding the trannys.
I have a feeling that power supply sag possibly does have an effect on the 'rails' growl but maybe we should at least start with your V3 and then go from there.
If power supply sag is part of that GK 'rails' sound then there is much more work to be done.
-Frank
In that case I'd love to be wrong!
